The mineral cristobalite is a high-temperature polymorph of quartz and tridymite. Polymorphism in Materials science is the ability of a solid material to exist in more than one form or Crystal structure Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Tridymite is a high-temperature polymorph of Quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal Triclinic crystals or scales It occurs as white octahedra in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of California and similar areas. Cristobalite is stable only above 1470 degrees Celsius, but can crystallize and persist metastably at lower temperatures. Metastability is a general scientific concept which describes states of delicate equilibrium
The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is "reconstructive", requiring the breaking up and reforming of the silica framework. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Tridymite is a high-temperature polymorph of Quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal Triclinic crystals or scales The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide These frameworks are composed of SiO4 tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO2. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the A tetrahedron (plural tetrahedra) is a Polyhedron composed of four triangular faces three of which meet at each vertex. A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy and may not happen on a human time frame. In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined Framework silicates are also known as tectosilicates. The silicate minerals make up the largest and most important class of rock-forming Minerals They are classified based on the structure of their silicate Ion group
There is more than one form of the cristobalite framework. At high temperatures the structure is cubic. The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube. A tetragonal form of cristobalite occurs on cooling below ca. In Crystallography, the tetragonal Crystal system is one of the 7 lattice Point groups Tetragonal Crystal lattices result from stretching a cubic 250 ªC at ambient pressure, and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or α − β transition. It may be termed "displacive"; i. e. , it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.
The cubic β-phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called rigid unit modes. In Physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the Atomic lattice of a Solid Rigid Unit Modes (RUMs represent a class of lattice vibrations or Phonons that exist in network materials such as Quartz, Cristobalite or Zirconium It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.
In the α–β phase transition only one of the three degenerate cubic crystallographic axes retains a four-fold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins can form within the same grain. Twins are Offspring resulting from the same Pregnancy, either of the same or opposite Sex. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.
When devitrifying silica, cristobalite is usually the first phase to form, even when well outside of its thermodynamic stability range. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.
The micrometre-scale spheres that make up precious opal are made of cristobalite, crystallized metastably at low temperature. OPAL ( Open Pool Australian Lightwater reactor) is a 20 megawatt pool-type nuclear research reactor that was officially opened in April 2007 Metastability is a general scientific concept which describes states of delicate equilibrium
References
- American Geological Institute Dictionary of Geological Terms.
- Durham, D. L. , "Monterey Formation:Diagenesis. In: Uranium in the Monterey Formation of California". US Geological Survey Bulletin 1581-A, 1987.
- Reviews in Mineralogy and Geochemistry, vol. 29. , Silica: behavior, geochemistry and physical applications. Mineralogical Society of America, 1994.
- R. B. Sosman, The Phases of Silica. (Rutgers University Press, 1965)
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